Microring resonators are a class of resonant filter that is receiving increased attention in the technical and business communities. Normally rig resonators include circular or elliptical cavities that can support standing wave resonant modes (i.e., also known as whispering gallery modes) for a wavelength of interest. If such a ring is placed near an optical waveguide which is conveying a stream of several closely-spaced wavelength channels, energy at the resonant wavelength of the ring can be coupled out of the stream without disturbing the other wavelength channels. Example uses for ring resonators include channel-dropping filters, channel-adding filters, non-linear optical applications and sensors.
FIG. 1 shows an example prior art channel-dropping filter 20. The filter 20 includes a ring resonator 22, an input waveguide 24 and an output waveguide 26. The ring 22 is separated from the waveguides 24, 26 by spaces 28. The ring 22 can support a standing wave resonant mode for a channel having a wavelength λ2. The ring 22 may also be able to support modes of different wavelengths as well. In use, a stream of closely-spaced wavelength channels (λ1, λ2, . . . λN) is conveyed through the input waveguide 24 in a direction indicated by arrow 30. As the stream of wavelength channels passes by the ring 22, the resonant wavelength λ2 of the ring 22 is separated from the stream of wavelength channels. Specifically, the wavelength channel λ2 is coupled out of the stream of wavelength channels λ1, λ2, . . . , λN and resonates about the outer surface of the ring 22 as indicated by arrows 32. The standing wavelength channel λ2 within the ring 22 is extracted from the ring 22 through the output waveguide 26 as indicated by arrow 34. The net result is that the information being conveyed on the channel λ2 is separated from the multichannel stream λ1, λ2, . . . , λN without disturbing the other wavelength channels of the stream.
FIG. 2 illustrates a prior art sensor 41 including a waveguide 40 and a resonator ring 42. The ring resonator ring 42 is separated from the waveguide 40 by a space 44. A substrate 46 is provided about the outer surface of the ring 42. The substrate 46 has characteristics which induce or encourage the attachment of a particular type of analyte (e.g., a biological species such as a bacteria or microbe, or a chemical species) which is desired to be defected. Prior to attachment of the analyte on the substrate 46, the ring 42 can support a standing wave resonant mode for a wavelength λ2. Thus, the ring 42 is capable of extracting a wavelength channel λ2 from a wavelength channel stream λ1, λ2, . . . , λN that is conveyed through the waveguide 40. When the analyte attaches (e.g., grows upon, chemically bonds with, or otherwise joins or adheres to) on the substrate 46, the resonant wavelength of the ring 42 shifts such that the ring 42 no longer can support a standing wave resonant mode for the wavelength λ2. When this occurs, the wavelength channel λ2 is no longer extracted from the channel stream. Instead, a different wavelength is extracted. Detection of this resonant wavelength shift or change in spectrum indicates, the presence of the analyte on the substrate 46.
While ring resonators are simple enough in principle, they can be difficult to fabricate because of the material and dimensional constraints required. A ring resonator frequently is required to have a ring radius in the neighborhood of 10–200 microns. The smoothness of the surface is most important for creating a precise resonance, and ring diameters required for setting the resonance wavelength, are often required to be fabricated and maintained to fractions of a micron. Precision in the separation between the ring resonator and the adjacent channel waveguides is also important to maintain the proper coupling between the ring and the channel. Again, the tolerances can be in the range of fractions of a micron.
The precise tolerances required in the manufacture of microring resonators places extreme demands on the processes used to manufacture ring resonators, especially in achieving the needed wall smoothness, ring dimensions and ring channel separations. Typical fabrication techniques involve planar microelectronic wafer processes such as thin film deposition, photolithography and etching. Because of the high aspect, ratio of the etch that is required, deep reactive ion etching techniques are typically used. Major drawbacks of this approach include the roughness of the etched sidewalls and the difficulty in maintaining precise ring/waveguide separation spacings. Roughness leads to a large amount of scattering loss which leads to broad resonance causing imprecise detection in sensors, and also creates poor optical coupling.